Sculpting planar concavities in zbrush

root

I'm trying to sculpt a plane head in Zbrush and I'm making pretty good progress for the most part with the trim and polish brushes, but an issue I tend to keep running into is getting nice clean junctions where the adjoining planes are concave - my brushes just dig into the facing plane and the best result i can get right now is just by smoothing the junction, which isn't really what I'm going for.   I can't seem to get a nice crisp transition, does anyone have an ideal technique for this?